1. Field of the Invention
The present invention relates generally to suspension of a vehicle seat, and more specifically to a suspension arrangement for a vehicle seat the hardness characteristics of which can be automatically adjusted in response to movement characteristics thereof relatively to the vehicle chassis on which it is mounted.
2. Description of the Prior Art
FIGS. 17 and 18 show a previously proposed suspension arrangement used to support a seat in a vehicle cabin or the like. This arrangement is disclosed in detail in British Patent Specification No. 1,230,734. As shown, a pair of springs 2,2 and a pair of dampers 4, 4 are disposed in a seat frame which is arranged essentially horizontally with respect to a frame or base.
One end of each of the springs 2, 2 and dampers 4, 4 is secured to a transverse connecting member 6 which extends between two pairs or scissor arms 8. The other ends of the dampers 4, 4 are connected to the seat frame 10 while the other ends of the springs 2, 2 are connected with a lever arrangement 12 which permits the tension in the springs to be adjusted. This arrangement, while permitting a very compact arrangement which can be readily applied to reclining seats has suffered from the drawback that as the springs 2, 2 and the dampers 4, 4 are operatively interconnected in a manner which permits the same to act directly against one and other, the damping period of the system tends to be excessively long, thus permitting the seat to oscillate up and down in a manner which imparts a disturbingly unstable sensation to the occupant. The arrangement also cannot be adjusted in a manner which selectively varies the hardness of seat suspension characteristics.
In order to overcome this problem it has been proposed in Japanese Patent Application 60-215127 filed on Sep. 30, 1985, one of three priority documents upon which United States application Ser. No. 912,801, now U.S. Pat. No. 4,913,482, is based. This U.S. Application by Hanai (one of the coapplicants of this application), was filed on Sep. 29, 1986, and the same is hereby incorporated by reference. This reference teaches a vehicle suspension system which includes a road condition sensing arrangement and means, such as variable setting shock absorbers for varying the hardness and damping characteristics of the suspension; and suspension of the seat on an arrangement wherein a variable setting type shock absorber is connected between the seat and the floor or chassis on which the seat is mounted. The shock absorber is provided with a valve arrangement which is switched between soft and hard settings synchronously with the setting of the suspension.
With this arrangement when the vehicle encounters a rough road, both the vehicle suspension and the seat suspension are simultaneously conditioned to produce "hard" characteristics. A drawback encountered in this approach is that the hardness of both suspensions causes vibrational energy transmitted to the chassis via the road wheels and the vehicle suspension to also be transmitted directly to the seat via the hard conditioned shock absorber of the seat suspension.
On the other hand, when the vehicle is traversing a smooth road, the seat suspension shock absorber is conditioned to produce soft characteristics at the same time as the vehicle suspension is so conditioned. Under these conditions, the seat suspension fails to maintain the level of the seat relative to the vehicle floor and permits the occupant to "float up and down" in a manner which disconcertingly induces a change in the person's eye point level.